This invention relates to recording members containing heat-reactive components and, more particularly, to recording members capable of us in thermographic copying, thermal printing, event recording, and as transparencies for overhead projection.
Heat sensitive sheets containing the cyclic polyketo compounds of this invention, useful for copying and recording and characterized by the ability to form a mark of contrasting color when heated to an activation temperature of 50.degree. C. are known in the art. They are used in thermographic processes wherein a recording member is positioned on a graphic original and exposed to infrared radiation to cause selective heating of the dark areas of the original sufficient to form a copy thereof on the heat sensitive member. The thermally responsive members have also been used to record the heated portions of a thermal print-head and also to record a colored trace when contacted by the hot stylus of a thermal recorder. The ninhydrinamine reaction wherein ninhydrin reacts with amino acids, primary amines, and certain derivatives of morpholine, piperidine, and pyrrolidine to give the dye commonly referred to as Ruhrman's purple is well known. Isatin reacts with these same amines to give isatin blue. Alloxan reacts with the amines to give a red dye. These reactions have been used in numerous inventions for thermally responsive copy and recording papers and films. Lawton, U.S. Pat. No. 3,736,166 used ninhydrin with various morpholine and piperidine derivatives to prepare transparencies for overhead projection. Lawton, U.S. Pat. No. 3,293,061 combined ninhydrin or hydrindantin with isatin-amine condensates to provide thermographic copy sheets. Bauman and Lawton reacted ninhydrin or hydrindantin with complexes of amines and flavans or phenolic compounds to make thermographic copy sheets, see U.S. Pat. No. 3,149,991 and U.S. Pat. No. 3,149,992. Huffman, U.S. Pat. No. 3,664,858 combined ninhydrin with the adducts of morpholine or piperidine and organic acids in thermal recording members. Sus. U.S. Pat. No. 3,024,362 combined hydrindantin with amino acids or salts of primary amines with organic carboxylic and sulfonic acids to make a thermocopy paper. Allen, U.S. Pat. No. 2,967,785 used the adducts of morpholine or piperidine with isatin or ninhydrin as the color forming material in thermocopy papers. Small, U.S. Pat. No. 3,573,958 combined an amine with a halide or organometallic halide of germanium, silicon, lead, and tin with hydrindantin to provide a heat-sensitive recording sheet. In each case the normal dye formations of Ruhrman's purple with ninhydrin, red dye with alloxan, and isatin blue were obtained.
The above thermally reacting color forming systems containing ninhydrin, isatin, alloxan, and their derivatives have a number of limitations. The most accepted commercial products use ninhydrin or hydrindantin as the color former. The image is purple, and has a tendency to fade with exposure to light and with time. It is desirable to have more acceptable image colors such as blacks, dark browns, and the like and improved resistance to image fading by light exposure or aging.
I have found that thermal images with more desirable colors and greater stability to light and aging can be obtained by using as the heat reacting material a cyclic polyketo color precursor and piperidine derivatives substituted at the 4 position with an aromatic group and a hydroxyl group of hydrogen atom. The substitutents at the 4 position cause a color shift from the purple normally obtained when piperidine and other amines are reacted with ninhydrin or hydrindantin to give dark shades of brown, black, blue-black, blue, etc. Similar color shifts are found when isatin derivatives were used to replace the ninhydrin.